In recent years, in order to address atmospheric pollution and global warming, there has been a strong demand for reduction of a carbon dioxide amount. In the automobile industry, there are high expectations that the reduction of carbon dioxide amount can be attained by introduction of an electric vehicle (EV) and a hybrid electric vehicle (HEV), and an electric device such as a secondary battery for driving a motor, which is the key to the practical realization of these vehicles, has actively been developed.
The motor-driving secondary battery is required to have a considerably high output property and high energy as compared to a consumer lithium ion secondary battery used in a mobile phone or a laptop personal computer. Therefore, a lithium ion secondary battery having the highest theoretical energy among all types of batteries has attracted attention and is being rapidly developed at present.
The lithium ion secondary battery generally has a structure in which a positive electrode obtained by coating a positive electrode active material and the like on both surfaces of a positive electrode current collector by using a binder and a negative electrode obtained by coating a negative electrode active material and the like on both surfaces of a negative electrode current collector by using a binder are connected to each other via an electrolyte layer and housed in a battery casing.
A carbon/graphite material which is advantageous in charge-discharge cycle life and cost has heretofore been used for the negative electrode of the lithium ion secondary battery. However, since the charge-discharge is performed by occlusion/release of lithium ions into/from a graphite crystal with the carbon/graphite negative electrode material, there is a drawback that it is difficult to attain a charge-discharge capacity of 372 mAh/g or more which is the theoretical capacity obtainable from LiC6 which is the largest-amount-lithium intercalation compound. Therefore, it is difficult to attain capacity and energy density which are satisfactory for the practical use in vehicles with the use of the carbon/graphite negative electrode material.
On the other hand, since a battery in which a material to be alloyed with Li is used for a negative electrode is expected as a negative electrode material for use in vehicles since the battery is improved in energy density as compared to the conventional carbon/graphite negative electrode material. For example, one mole of a Si material occludes and releases 4.4 mol of lithium ions as shown in a reaction formula (I), and a theoretical capacity of Li22Si5 (=Li4.4Si) is 2100 mAh/g. Further, in the case of calculation per weight of Si, an initial capacity of 3200 mAh/g (see Sample 10 of Example 1) is attained.[Formula 1]Si+4.4Li++e−Li4.4Si  (1)
However, in the lithium ion secondary battery using the material alloyed with Li for the negative electrode, expansion-shrinkage in charge-discharge is large in the negative electrode. For example, a volumetric expansion of a graphite material in the case of occluding Li ions is about 1.2 times, while the Si material has a problem of reducing a cycle life of an electrode due to a large volumetric change (about 4 times) which is caused by transition from an amorphous state to a crystal state in the alloying between Si and Li. Also, since a capacity and cycle durability have a trade-off relationship in the case of the Si negative electrode active material, there has been a problem that it is difficult to improve high cycle durability while maintaining a high capacity.
In order to solve the problems, a negative electrode active material for lithium ion secondary battery, which contains an amorphous alloy having a formula of SixMyAlz, has been proposed (see Patent Document 1, for example). In the formula, each of x, y, and z represents an atomic percent, x+y+z=100, x≧55, y<22, z>0, and M is a metal formed of at least one of Mn, Mo, Nb, W, Ta, Fe, Cu, Ti, V, Cr, Ni, Co, Zr, and Y. In the invention disclosed in Patent Document 1, there is the description in the paragraph [0018] that good cycle life is exhibited in addition to a high capacity by minimizing the content of the metal M.